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The Origins
of the Storm

by
on January 20, 2010 4:02 PM

Because of the jet stream.

At about 30,000 feet (approximately where commercial jets fly), a band of fast moving, low pressure air above the northern hemisphere flows over large cells of low pressure and around high pressure cells.

Given direction by this invisible and mobile topography, the jet stream loops north and south and meanders east and west. And as the jet stream moves, low pressure systems below it – associated with disturbed air and storms – follow. In a planetary feedback loop, the faster the winds in jet stream above, the more troubled the air below.

The cycle of storms in the Los Angeles basin reflects a deep southern bend in the jet stream, which would normally flow along the North American coast. The “L” shape of the flow propels the associated low pressure cells to make a long pass over Pacific waters. These waters are warmer this year (the El Niño effect). The low pressure cells – warmer and wetter from their Pacific passage – move east with the jet stream. These cells bear a greater than normal capacity to deliver rain.

But that’s not entirely why it’s raining now (or why the LA basin has been repeatedly flooded).

The warm, wet air delivered by the jet stream slams into the ridges and peaks of the mountains north and east of LA. The wet air rapidly cools as it is forced upward. Cooler air can carry less moisture than warm air. And so it rains.

The combination of jet stream meandering, long passage over warmer seas, and the mountains surrounding Los Angeles are making it rain.

For a detailed explanation of these causes and their effects, go here.

The image on this page is by Flickr user AlmazUK. The image is used under a Creative Commons license.

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